Polarisation Curves and Experiment Design as Tools in the Search of Optimised Inhibitors Mixture Formulation for HSLA Steel in Hydrochloric Acid

Abstract

Corrosion inhibitors are intentionally added to pickling baths in order to prevent metal dissolution during its surface cleaning from oxides and other adherent materials. The main goal of this work is to verify if there is any synergism when the three following inhibitors: benzotriazole (BTAH), N-phenylthiourea (N-PTU) and hexadecyltrimethylammonium chloride (HTAC) are put together in hydrochloric acid (HCl) medium to prevent high strength low alloy (HSLA) steel corrosion and to find an optimised formulation that yields the highest HSLA steel surface protection degree from weight loss measurements and from anodic and cathodic potentiodynamic polarisation curves. The experiment design used was a simplex-lattice type which yields the Sheffe polynomial expressions as the model, with a 16 experiments matrix and the inhibitors concentrations as independent variables and the steel surface protection degree as response or dependent variable for a complete cubic model. A cubic model was obtained from regression analysis with a determination coefficient R 2 = 0.98 and a good agreement between experimental results and the predicted values from the model. The cubic model coefficients demonstrate that there is a significant synergism between BTAH and N-PTU and also between N-PTU and HTAC and a optimised formulation was found. The potentiodynamic curves obtained for inhibitors concentrations that have showed synergism were in close agreement with the results of weight loss measurements from the experiment design.